Antigen contacts by Ni-reactive TCR: typical alpha beta chain cooperation versus alpha chain-dominated specificity

VB17(+) TCR dominate in Ni-driven T cell cultures from highly Hi-sensitized patients. Using transfection of TCR from three CD4(+), VB17(+), Ni-specifi c human T cell clones, we studied their Ni-MHC contacts by site-directed TC R mutation and combination of alpha and beta chains between different TCR. All three TCR exhibited N-nucleotide-determined Arg-Asp motifs in their CDR 3-beta sequences. Two of them were specifically restricted to HLA-DR13, whi le the third one accepted a variety of HLA-DR alleles, The highly similar a lpha or beta chains of the DR13-restricted TCR were interchangable without loss of specificity, but alpha or beta chains of other TCR were not tolerat ed. Mutations of their Arg-Asp motif revealed loss of reactivity upon excha nging Asp for Glu or Ala and of Arg for Ala but not of Arg for Lys or the H i binding His. Reactivity was also destroyed by mutation of a chain positio n 51, proposed as a general contact site for MHC, Hence, in these two TCR t he Arg-Asp motif is clearly involved in contacting HI-MHC complexes, and cl ose cooperation between alpha and beta chain is required. In contrast, the third TCR retained Hi reactivity upon mutation of a chain position 51 or of its beta chain Arg-Asp motif, which rather affected the pattern of DR cros s-restriction, Moreover, its alpha chain paired with various beta chains fr om other, even mouse TCR, irrespective of their specificity, retaining Hi r eactivity as well as promiscuous HLA-DR restriction. This preponderance of an a chain in defining specificity indicates fundamental differences in Hi interactions of individual TCR and implies that beta chain similarities may not necessarily result from antigen selection.